Your concern is legitimate, but I think you can count on one (and probably both) of two things happening to protect you:
(1) Amazon or others will sell replacement devices or software that support the same DRM'd formats, so you will have continuity of access. (For example, since iPhones can also read Kindle books, and presumably Android phones too at some point, there will be multiple reading platforms.)
(2) The progress of time and mores will render the DRM effectively inert, as with all other DRM formats used on popular/mass content.
There is a bit of faith required, but not much -- that one or both of these occur fits a trend as strong as Moore's Law.
(1) Amazon or others will sell replacement devices or software that support the same DRM'd formats, so you will have continuity of access. (For example, since iPhones can also read Kindle books, and presumably Android phones too at some point, there will be multiple reading platforms.)
(2) The progress of time and mores will render the DRM effectively inert, as with all other DRM formats used on popular/mass content.
There is a bit of faith required, but not much -- that one or both of these occur fits a trend as strong as Moore's Law.